The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the reduction of image artifacts caused by signals produced outside the field of view.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
To accurately excite spins and resolve the locations of the resulting NMR signals the polarizing magnetic field B0 must be highly homogeneous and the imaging gradient fields Gx, Gy and Gz must be highly linear. Numerous structures and methods are known in the art to accomplish this in commercial MRI system, and the region where these fields meet the requirements is referred to as the designed spherical volume (“DSV”). The DSV may range for example, from a diameter of 40 to 48 cm. Outside the DSV, the polarizing magnetic field B0 can become very inhomogeneous and the imaging gradients Gx, Gy and Gz can become highly nonlinear. They are also very poorly controlled in these outer regions.
Referring particularly to FIG. 2, the DSV of a typical MRI system is indicated by dashed line 10 and a subject to be scanned 12 is placed in the DSV 10. A field of view (FOV) from which accurate NMR data is acquired to reconstruct an image is indicated by dotted lines 14. Portions of the subject 12 are outside the DSV 10, and the spins therein are subject to the RF excitation fields and magnetic fields produced by the MRI system while imaging the FOV 14. The NMR signals produced by spins located outside the DSV 10 can produce image artifacts. These image artifacts from outside the DSV 10 can be aliased into the reconstructed image because of the limited imaging FOV 14, they can be depicted in the FOV 14 due to system imperfections, or they can also be ghosted into the image because of the data inconsistency.
Methods and apparatus are known to reduce these artifacts. One solution is to increase the imaging FOV 14 to reduce aliasing. Hardware solutions include design of gradient coils with a larger linear region or RF transmit coils which significantly reduce RF excitation of spins outside the DSV 10. These are costly solutions which require major system changes.
Another well known method for suppressing artifact-producing signals emanating from spins located outside the FOV 14 is to interleave spatial saturation pulse sequences with the imaging pulse sequences. As described in U.S. Pat. No. 4,175,383, a spatial saturation pulse sequence suppresses the longitudinal magnetization of the spins in a selected slice or slab outside the FOV 14 by applying a selective RF excitation pulse in the presence of a slice select gradient to excite spins in the selected slice. A spoiler gradient is then applied to dephase the resulting transverse magnetization. Before the longitudinal magnetization in the excited slice can recover, imaging data is acquired from the FOV 14. Because the longitudinal magnetization in the excited slice is suppressed, very little artifact producing signal can be produced in the presaturated slices during the subsequent imaging pulse sequence.
The effectiveness of the spatial saturation method depends on homogeneous B0 and B1 fields and linear gradient fields in the regions outside the FOV 14 to accurately locate the spatial saturation slice and adequately suppress the spin signals therein. Since the regions outside the DSV 10 do not necessarily satisfy these conditions, the spatial saturation method can be ineffective on any given MRI system in any given location depending on the peculiarities of its fields outside the DSV 10.